Pellet burning heating devices are known such as those described in the following patent(s) and publications(s):

Pellet stoves are often installed in residential homes as secondary environmental air heat sources and, in some instances, as a primary environmental air heat source. A pellet stove is an automated burner that burns economical pellets made typically from recycled sawdust and other forms of biomass such as corn. The biomass material (often biomass waste material) is processed into generally consistent sized and geometrically shaped solid units as in pellets. A typical pellet stove has an electro-mechanical, automated fuel feed system which delivers the wood pellets from an integrated feed hopper to a burn chamber. Once pellets are ignited, combustion air is forced through the burning pellets with an electric fan creating a mini-furnace. A typical pellet stove normally consists of a hopper, a screw auger system, a blower system (often a combustion fan, convection fan and an exhaust fan), a firebox (with refractory panels, a burn pot with apertured wear plate), and an operation control system (which, for example, runs the fans, feed auger and monitors for safety issues). Often there is provided a front door (often a glass windowed door) for access (e.g., cleaning ash from the burn pot) and an underlying ash tray. The automated feed and control system is designed to provide for a high degree of combustion of the fuel and relatively consistent heat energy output.
Fluid heat exchange systems are also featured in the art such as those described in the following patent(s) and publication(s):

There is lacking in the art a domestic hot water heater system that takes advantage of the characteristics of regulated heat supply heating devices as in a pellet stove relative to a heat absorption component as in a domestic hot water unit (e.g., a 20 to 90 gallon domestic hot water tank) via an efficient circulation loop and flow control system. There is also lacking in the prior art a hot water heater system that provides for rapid retrofitting of a preexisting hot water heater system, without substantial deviation of that system, for use with a pellet stove or the like, as well as increasing the options as to a multitude of different fuel sources for a domestic hot water tank or the like.